1. Field of the Invention
The present invention relates to a technique for controlling wavelength characteristics of optical transmission powers caused in wavelength division multiplexed signal light to be transmitted through an optical transmission path, and particularly to a method for compensating the wavelength characteristics of optical transmission powers making use of Raman amplification, as well as to a wavelength division multiplexing optical communication system and an optical amplifier to which the controlling method is applied.
2. Related Art
Wavelength division multiplexing (WDM) optical transmission system, such as making use of a wide gain band of an Erbium doped fiber amplifier, is capable of increasing a communication capacity by transmitting an optical signal including a plurality of wavelengths through a single optical fiber. This type of WDM optical communication system has such advantages that the system can be inexpensively introduced by utilizing existing optical fibers, and its transmission path is made to be bit-rate free by utilizing such as optical amplifier thereby facilitating future upgrade.
To obtain required transmission characteristics in a WDM optical communication system, it is necessary to restrict dispersions of light powers between channels to 1.0 dB or less at respective optical repeating stages. This is because an upper limit of light power is restricted by nonlinear effect of a transmission path, and a lower limit thereof is restricted by a receiving S/N due to spontaneous emission light of an optical amplifier. As such, it is required to diminish wavelength loss characteristics such as of transmission path and dispersion compensation fiber constituting a WDM optical communication system. Particularly, for an optical amplifier which collectively amplifies multi-wavelength optical signals and serves as a key component of a system, it is strongly desired that output deviations among channels be low in addition to that the optical amplifier transmits a predetermined output level with a low noise characteristic.
In an actual WDM optical communication system, there are caused wavelength characteristics of optical transmission powers among respective channels, such as due to the following matters:
{circle around (1)} wavelength loss characteristic of transmission path, due to Rayleigh scattering;
{circle around (2)} wavelength loss characteristic of dispersion compensator;
{circle around (3)} wavelength loss characteristic of transmission path due to induced Raman scattering;
{circle around (4)} wavelength characteristic of gain of optical amplifier; and
{circle around (5)} temperature characteristics of transmission path, of dispersion compensator and of optical amplifier.
Concretely, such as in case of adopting a 1.3 μm zero-dispersion single mode fiber (SMF) of a length of 80 km provided that a wavelength band of signal light is between 1530 to 1560 nm, deviations of optical transmission powers caused due to the matters {circle around (1)} and {circle around (3)} are approximately 0.5 dB and approximately 1 dB, respectively. Deviation to be caused by the matter {circle around (2)} is approximately 0.5 dB when a general dispersion compensation fiber (DCF) is adopted as a dispersion compensator, while deviation to be caused by the matter {circle around (4)} is approximately 1 dB when a general Erbium doped optical fiber amplifier (EDFA) is adopted. Further, deviation to be caused by the matter {circle around (5)} can be evaluated as being approximately 0.3 dB when adopting the aforementioned optical devices.
The thus caused wavelength characteristics have different magnitudes depending on transmission conditions (such as number of channels, channel separations, input power, transmission path length). As such, it is necessary to newly or additionally apply a means capable of compensating the variably caused wavelength characteristics, to a WDM optical communication system. When wavelength characteristics of optical transmission powers among channels are to be compensated at each of optical amplifying and repeating stages provided in a WDM optical communication system, a width or magnitude of wavelength characteristic to be compensated at one optical amplifying and repeating stage is approximately 3 dB.
Conventionally, there has been proposed a method in which such as variable gain equalizers capable of varying wavelength loss characteristics are newly applied to an optical communication system, as a scheme for compensating wavelength characteristics of optical transmission powers as described above. According to such a method, optical transmission powers among channels are equalized by controlling wavelength loss characteristics of variable gain equalizers corresponding to respective wavelength characteristics of optical transmission powers caused in the system.
However, in the aforementioned conventional method for controlling wavelength characteristic of optical transmission powers, the optical transmission powers are equalized by giving losses to respective channels other than a channel having a minimum power in a manner matching to the minimum power channel. As such, there is a possibility that an optical S/N ratio is degraded and transmission characteristics are thereby deteriorated, as compared to the time such as before insertion of variable gain equalizers.
As one method for compensating wavelength characteristics of optical transmission powers while restraining loss to a smaller value, it is effective to utilize Raman amplification. This method is adapted to compensate wavelength characteristics of optical transmission powers, by preferentially amplifying channels of lower powers.
As techniques utilizing Raman amplification, there are known articles of, for example, S. Kinoshita et al., OECC, 10B2-3, July, 1997; and Emori et al. entitled “A broadband dispersion compensating Raman amplifier pumped by multi-channel WDM laser diodes”, Technical Report of IEICE, OCS98-58, pp. 7-12, 1998. The techniques described in these articles have contemplated realizing lower loss of dispersion compensation fiber and broader band of optical amplifier, by Raman amplifying a dispersion compensation fiber within an optical amplifier by utilizing a pump light source such as at 1,480 nm band. Further, in Japanese Unexamined Patent Publication No. 10-73852, there is described an optical amplifying transmission system which has contemplated realizing a broader band of signal transmission, making use of Raman amplification.
However, in these known techniques utilizing Raman amplification, Raman amplification is generated by supplying fixed pump light of which wavelength band and power have been previously set, such as to a dispersion compensation fiber. Thus, when fluctuation has occurred in wavelength characteristics of optical transmission powers such as caused in a transmission path, it is difficult to flexibly change a gain wavelength characteristic of Raman amplification corresponding to such fluctuation. In a WDM optical communication system, since it is considered that such as the number of channels of WDM signal light to be transmitted and wavelengths to be used are changed variously, and that wavelength characteristics of optical transmission powers fluctuate, it is accordingly desired to flexibly compensate wavelength characteristics corresponding to such fluctuation.